BELFAST, Northern Ireland (AP) — Extremists still scheming to spoil Northern Ireland’s fragile peace can be beaten through a commitment to political sacrifice, compromise and vigilance, U.S. Secretary of State Hillary Clinton said Friday as she visited a Belfast torn by rising sectarian passions and death threats.

Clinton stood shoulder to shoulder with the leaders of Northern Ireland’s 5-year-old unity government to denounce this week’s violence and intimidation fueled by arguments over flying the British flag.

“Peace does take sacrifice and compromise and vigilance, day after day. We’ve seen that again this week, that the work is not complete, because we have seen violence break out again,” said Clinton, flanked by First Minister Peter Robinson, a British Protestant, and Deputy First Minister Martin McGuinness, an Irish Catholic and former Irish Republican Army commander.

“All parties need to confront the remaining challenges of sectarian division peacefully and together,” she said.

Yet even as Clinton spoke, police in Northern Ireland’s second-largest city Londonderry announced they had seized an armor-piercing rocket system used to blast passing police patrols, who still use armor-plated Land Rovers.

The weapon was found in a car rammed by police Thursday night in an IRA power base in the predominantly Catholic city 80 miles (130 kilometers) northwest of Belfast. Police arrested three suspected members of an IRA splinter gang in the car and a fourth man nearby.

The city’s police commander, Chief Superintendent Stephen Martin, said he was “delighted that we have been able to take (the weapon) out of circulation before it was used for its deadly purpose.”

IRA die-hards, who are rooted in the toughest Catholic districts, continue to plot attacks in defiance of the mainstream IRA’s 2005 decision to renounce violence and disarm following a failed three-decade campaign to force Northern Ireland out of the United Kingdom.

At the other extreme of Northern Ireland opinion, Protestants opposed to any compromise with the Catholic minority have mounted a series of illegal, violent protests this week against Belfast City Council’s vote Monday to stop flying the British flag year-round, a policy in force for the past century.

Much of the Protestant anger has focused on Alliance, a small mixed-religion party that seeks to promote compromise between the two camps. It sided in the vote with Catholic politicians, who wanted the flag completely removed but agreed to let it fly 18 days a year.

Alliance’s sole member of the British Parliament, Naomi Long, said police told her not to sleep in her home overnight or to go to her office Friday. The warning followed arson attacks on two Alliance offices Wednesday and a paint bomb attack on the home of two other Alliance politicians early Thursday. Protestant militants also tried to storm Belfast City Hall on Monday night following the flag vote; 15 policemen and an Associated Press photographer were wounded.

Long, who met Clinton as part of a multi-party delegation of female politicians, said she wouldn’t be forced from her home or work. “I trust to God for my safety and security,” she said. “I will not allow this to stop me.”

At their joint news conference, Robinson and McGuinness lauded Clinton and her husband for their exceptional two-decade commitment to brokering peace in Northern Ireland, a land of 1.7 million people, and ending four decades of conflict that have claimed 3,700 lives.

“We do know that there are people on the extremes of (British) loyalism, and people on the extremes of (Irish) republicanism, who wish to plunge us back to the past. We’re going to resist that with every fiber of our being,” said McGuinness, a longtime senior figure in the dominant IRA faction, the Provisionals, which killed nearly 1,800 people from its 1969 foundation to its 1997 cease-fire.

He, Robinson and Clinton all condemned the threat to Long.

But their appeal for calm appeared to fall on deaf ears as Protestant hard-liners mounted several more illegal road blocks across Belfast on Friday night, snarling rush-hour traffic.

After dark, one conflict zone on Donegall Road in south-central Belfast descended into a full-blown riot after youths attacked a van that tried to force its way through the crowd. Riot police deployed mobile water cannons against hundreds of teens and young men, many of whom used hoods and scarves to conceal their faces from police cameras. Rioters there also hijacked and set fire to at least one vehicle.

Police braced for potentially worse trouble Saturday as protesters vowed to take their angry demonstrations into downtown Belfast on one of the busiest Christmas shopping days.

During her Belfast visit, Clinton also visited its major new tourist attraction, a 100 million pound ($160 million) dockside center devoted to the Titanic, the ill-fated luxury liner built in the city a century ago.